Trehalose (a-D-glucopyranosyl-[1,1]-a-D-glucopyranoside) is a disaccharide commonly found in lower organisms such as bacteria, fungi and insects where it often accumulates in resting or stationary phase cells and organs. Two enzymatic activities are required for trehalose biosynthesis: a trehalose-6-phosphate synthase catalyses the condensation of UDP-glucose and glucose-6-phosphate to trehalose-6-phosphate and a trehalose-6-phosphate phosphatase phosphorylates trehalose-6-phosphate to trehalose.
Although trehalose can serve as a storage form for reduced carbon, it may play a more significant role as a protectant against the deleterious effects of various abiotic stresses, notably heat and desiccation. Both in vivo and in vitro, trehalose accumulation is correlated with protection of biological macromolecules (particularly membranes and proteins) from dessication, temperature extremes, and osmotic shock. Trehalose produced by fermentation is used commercially in the preservation of enzymes and is registered as a food additive for the stabilization of dehydrated and processed foods.
While it has long been recognized that trehalose may occur in plants as a product of symbiotic microorganisms, as a rule vertebrates and higher plants were thought not to be capable of synthesizing trehalose. The near-ubiquitous occurrence of specific trehalose-catabolizing enzymes (trehalases) in higher plant families was a biological curiousity ascribed mainly to the presence of exogenous trehalose entering plant cells from symbiotic or epiphytic microbial and fungal sources. Notable exceptions are the lower plants and angiosperms grouped loosely into the category of “resurrection plants” which are capable of surviving extraordinarily prolonged periods of dessication. These plants, including species of Selaginella and Myrothamnus, can accumulate as much as 10% trehalose by dry weight following the onset of droughting.
In view of trehalose's association with drought resistance and the historically poor economics of microbial trehalose fermentation, attempts have also been made to engineer transgenic plants to accumulate this disaccharide. Although such plants have been obtained, it has become apparent that constitutive trehalose production in the plant cytosol is accompanied by significant deleterious effects. These phenotypes (stunted growth, abnormal leaves, undeveloped roots) are particulary severe when trehalose expression occurs in root tissue or during early development, as the use of green-tissue specific plant promoters to drive trehalose producing genes ameliorates some, but not all, of these effects.
Given these facts, an inducible expression system for the trehalose biosynthetic genes, which allows for trehalose accumulation and results in drought resistance but without deleterious effects to the plant is of great practical use and economic interest.